Neo-intimal hyperplasia represents an increase in the number of smooth muscle cells (SMC's) between the endothelium and the internal elastic lamina of a blood vessel. When intimal hyperplasia occurs, de novo thickening of the intimal layer, or vessel wall may result, causing the vessel to become stenosed, or occluded. Proliferation of arterial SMC's commonly occurs when a blood vessel is deformed, or disturbed during surgery. Surgical anastomoses, in particular with bypass grafts (in which a vein or synthetic substitute is anastomosed to an artery), may result in SMC proliferation and, consequently, stenosis.
When an obstruction in a blood vessel has been cleared, intimal hyperplasia occurring after such intervention may lead to the vessel's becoming occluded again. This is known as re-stenosis and may require further intervention.
Atherosclerotic cardiovascular disease is the leading cause of death in Europe and North America and is caused by occlusion of the arterial lumen, either preventing, or reducing blood flow. Depending on site and disease distribution, several options for treatment exist, with arterial bypass grafting being common.
A significant number of arterial bypass grafts fail, i.e. become occluded, in the first two years following surgery. In these cases, it is SMC intimal hyperplasia that often is responsible for causing stenosis of the arterial lumen, eventually resulting in complete occlusion. SMC intimal hyperplasia occurs most commonly around the more distal anastomosis and in the “native” vessel wall opposite the anastomosis. SMC intimal hyperplasia can occur at the proximal arterial anastomosis also and along parts of the graft itself.
A graft is commonly anastomosed to the native vessel in one of three ways: “end-to-end”, “end-to-side”, or “side-to-side”. Of these techniques, end-to-side and side-to-side are more common than end-to-end.
U.S. Pat. No. 3,626,947 describes a method and apparatus for applying a textile mesh reinforcing tube around a vein or artery. U.S. Pat. No. 4,743,251 describes a perforated flexible sheath for use in coronary bypass procedures.
EP-A-0730849 (and GB-A-2298577) disclose a non-restrictive, porous stent for use in arteriovenous bypass grafting procedures. The porous stent, when placed around a bypass vein segment, has beneficial effects on the luminal size and degree of medial and intimal thickening of the anastomosed vessel. It is apparently essential that the inner diameter of the stent should be at least 3 mm larger than the outer diameter of the expanded vein (which can only be determined in use). This specification focuses on the problems associated with end-to-end anastomoses.